Single and dual band 77/95/110GHz metamaterial absorbers on flexible polyimidesubstratePramod K. Singh, Konstantin A. Korolev, Mohammed N. Afsar, and Sameer Sonkusale

Citation: Applied Physics Letters 99, 264101 (2011); doi: 10.1063/1.3672100 View online: http://dx.doi.org/10.1063/1.3672100 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/99/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Micro-electro-mechanically switchable near infrared complementary metamaterial absorber Appl. Phys. Lett. 104, 201114 (2014); 10.1063/1.4879284 Broadband polarization-insensitive absorber based on gradient structure metamaterial J. Appl. Phys. 115, 17E523 (2014); 10.1063/1.4868090 Towards left-handed metamaterials using single-size dielectric resonators: The case of TiO2-disks at millimeterwavelengths Appl. Phys. Lett. 101, 042909 (2012); 10.1063/1.4739498 Performance enhancement of terahertz metamaterials on ultrathin substrates for sensing applications Appl. Phys. Lett. 97, 261909 (2010); 10.1063/1.3533367 Dual band terahertz metamaterial absorber: Design, fabrication, and characterization Appl. Phys. Lett. 95, 241111 (2009); 10.1063/1.3276072

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Single and dual band 77/95/110 GHz metamaterial absorbers on flexiblepolyimide substrate

Pramod K. Singh,1 Konstantin A. Korolev,2 Mohammed N. Afsar,2

and Sameer Sonkusale1,a)

1Nano Lab, Department of Electrical and Computer Engineering, Tufts University, Medford,Massachusetts 02155, USA2High-Frequency Materials Measurement and Information Center, Department of Electrical and ComputerEngineering, Tufts University, Medford, Massachusetts 02155, USA

(Received 29 August 2011; accepted 2 December 2011; published online 27 December 2011)

Ultra thin millimeter-wave absorbers on flexible polyimide substrate utilizing metamaterials are

implemented for single and dual frequency bands in an emerging frequency spectrum of 77, 95,

and 110 GHz. The dual band absorber is designed using a novel approach of imbedding high fre-

quency resonator inside low frequency resonator capable of absorbing electromagnetic energy at

both 77 and 110 GHz bands simultaneously. The total thickness of the absorber is just 126 lm

(almost 1/20th of the wavelength). Measured peak absorptions for single frequency absorbers are

92, 94, and 99% at 77.2, 94.8, and 109.5 GHz, respectively, and for dual band absorber 92% at

77 GHz and 94% at 109.8 GHz. VC 2011 American Institute of Physics. [doi:10.1063/1.3672100]

Metamaterials are made by inclusion of sub-wavelength

metallic structures in host dielectric medium, engineered to

achieve unusual properties not found in the nature. Recently,

metamaterials enabled design of thin electromagnetic energy

absorbers.1 Compared to conventional millimeterwave

absorbers which are physically thick and frequency perform-

ance is dictated by the inherent complex permittivity2 and

permeability3,4 of the bulk material, metamaterial based

absorbers can be tailored for frequency response through ge-

ometry of metallic inclusions. Metamaterial absorbers are

frequency selective and have already been investigated over

wide range of frequencies such as microwaves,1 THz,5,6 IR,7

and optical.8 Absorbers have many applications in the radar

imaging,9 wireless communication,10 thermal imagers,11 and

solar cells.8 Making metamaterials on flexible substrates12

allows for conformable and flexible applications such as mit-

igating multipath effects in radome,13 suppressing parasitic

coupling in antenna array,14 reducing scattering noise in

automotive radar15 and reducing electromagnetic coupling in

systems.2,16 Recently, dual frequency band absorbers17 have

also been designed using metamaterials with different reso-

nator geometries.5,6 Such dual band absorbers can be used in

the dual band transceiver systems, enhanced energy absorp-

tion for imaging, and for enhanced chemical and biological

sensing.

We present metamaterial absorbers at millimeter-wave

frequencies of 77, 95, and 110 GHz on the flexible polyimide

substrate for the first time. These millimeter-wave bands are

used for different applications such as automotive radar

(77 GHz), high speed point-to-point local area wireless net-

work, point-to-multipoint distribution, space born radios (92-

95 GHz), inter satellite link, and imaging (95, 110 GHz).

Additionally, we also demonstrate a dual band absorber

which absorbs electromagnetic energies at both 77 and

110 GHz frequencies using novel imbedding of one resonator

inside another.

The metamaterial absorber (shown in Fig. 1) consists of

dielectric substrate patterned with metallic split ring resona-

tors (SRRs) on one side and continuous ground metal on the

other side. The incident electromagnetic wave interacts with

SRRs of the sub-wavelength dimensions. At resonance fre-

quency of the metamaterial resonators, the incident electro-

magnetic energy is strongly coupled and dissipated in the

substrate and metal due to dielectric and ohmic losses. This

results in absorption of energy at resonance frequency. The

scattering of the energy is considered negligible and there is

zero transmitted energy through the sample due to metallic

backplane. At frequencies other than the resonant frequency,

the incident electromagnetic wave is reflected and no energy

is absorbed.

In this paper, a flexible polyimide substrate with thick-

ness (h) of 125 lm is used as dielectric substrate. Relative

permittivity and loss tangent of the substrate determined by

measurements for 70-120 GHz band are 3.2 and 0.01, respec-

tively. Absorbers are designed and simulated using FDTD-

electromagnetic simulation in CST Microwave Studio

(2010) software. The planar metamaterial is simulated using

a single unit cell of metamaterial with periodic boundary

conditions. For this design, the incident electromagnetic

wave is expected to be normal to the metamaterial absorber

surface. Due to negligible transmission on backside, the

evaluation of this particular metamaterial absorber can be

performed by considering it as single port system in simula-

tion and measurements.

Although several different geometrical constructs for

unit cells are possible, a single polarization “C” shaped SRR

metamaterial is used in the proposed design. The desired

electric field polarization is parallel to the gap direction of

the SRR (as shown in Fig. 1(a)) for the maximum interaction

with the electric field of the incident wave. The wave with

electric filed polarization perpendicular to the gap of the

SRR is reflected back completely and not absorbed. The dual

band absorber is implemented using higher frequency reso-

nator imbedded inside the lower frequency resonator. Hence,a)Electronic mail: sameer@ece.tufts.edu.

0003-6951/2011/99(26)/264101/4/$30.00 VC 2011 American Institute of Physics99, 264101-1

APPLIED PHYSICS LETTERS 99, 264101 (2011)

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additional area is not required for the placing second resona-

tor in the lattice, compared with prior work that used wallpa-

per geometry which suffers from poor fill factor.6 The

proposed imbedding approach increases fill factor and effec-

tive absorptivity per unit area for both frequency bands. Two

resonators resonate at two different frequencies and contrib-

ute to absorption of the energy at those frequencies. One

could easily extend this approach for multiple bands by

imbedding more structures.

The simulated power reflection coefficient (R¼ reflected

power/incident power) and power transmission coefficient

(T¼ transmitted power/incident power) of 77 GHz metama-

terial absorber are presented in Fig. 2(a). The simulated Rand T coefficients are calculated from the simulated S-pa-

rameters as; R¼ S112, T¼ S21

2. The power absorption coeffi-

cient (A) is estimated using A¼ 1-R-T. Since wave energy is

neither reflected nor transmitted (R¼ 0, T¼ 0) hence highly

absorbed (A¼ 1) at resonance frequency. This absorbed

energy is dissipated by dielectric loss of the substrate and

resistive loss of the metal. The simulated power loss density

of the absorber at peak absorption frequency of 77 GHz is

shown in inset of Fig. 2(a). Most of the power is lost in gap

region of the SRR due to high confinement of the electric

field. As it can be seen from the simulated results of Fig.

2(a), the absorption occurs for a single polarization when

electric field is polarized parallel to the gap. However, wave

is reflected back when polarization is perpendicular to the

resonator gap direction indicating no absorption. Hence,

metamaterial absorber can be made frequency selective and

polarization dependent which is not possible in conventional

bulk material based absorber.

The dual band absorber implemented in this research

uses two resonators one embedded inside another. As pre-

sented in Fig. 2(b), each resonator absorbs energy separately

at their resonance frequencies. In the imbedded geometry of

two resonators, the combination is capable of absorbing

energy at both frequencies. Only a minor shift in resonance

frequency and change in reflection coefficient of one resona-

tor are observed due to the presence of other resonator. This

could be explained through presence of some coupling, albeit

FIG. 1. (Color online) (a) Schematic rep-

resentation of metamaterial absorber. The

incident wave is reflected back with no

transmitted wave due to backside metal

layer. The wave is absorbed at resonance

frequency when electric field is parallel

to the gap of the SRR as shown in the fig-

ure. Layout of the metamaterial array for

(b) single band and (c) dual band absorb-

ers. The dimension of the unit cell is

square in shape and their periodical distri-

bution forms 2D square lattice.

FIG. 2. (Color online) Simulated results

of the absorbers: (a) single band

absorber at 77 GHz, showing power

reflection, transmission, and estimated

absorption coefficients for the electric

field polarization parallel to the gap of

(SRR). Also, the reflection coefficient

for the perpendicular electric field polar-

ization is presented, showing complete

reflection of the wave energy. (b) Dual

band absorber: power reflection coeffi-

cient of individual resonators (dotted

lines) and response when imbedded

(continuous lines). Power loss density of

absorbers is shown in the inset; resona-

tors are represented with false colors.

FIG. 3. (Color online) Comparison of

simulated (continuous lines) and mod-

eled (dotted lines) power reflection coef-

ficient: (a) single band absorber and (b)

dual band absorber showing individual

resonators and imbedded resonator

response with mutual inductive coupling

(M¼ 0.07).

264101-2 Singh et al. Appl. Phys. Lett. 99, 264101 (2011)

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131.156.59.191 On: Mon, 08 Sep 2014 11:29:53

small. An additional iteration in the design flow is executed

to change the dimensions of resonators to adjust for fre-

quency shift. The power loss density of the dual band

absorber is presented in inset of Fig. 2(b). It can be verified

that the loss signifying absorbed power mainly occurs in the

respective resonators at the two frequencies.

The absorption phenomenon in metamaterial can be

explained by considering simultaneous interaction of the

electric (in split gap of SRR) and magnetic fields (between

SRR and back metal) of the electromagnetic wave with

metamaterial.1 At the resonance frequency, the ratio of elec-

tric and magnetic field (E/H) known as wave impedance

(g¼E/H¼Hl/e) should be equal to the free space wave im-

pedance (g0¼Hl0/e0) to provide impedance matching as a

criteria for zero reflection. The extraction of frequency

dependant effective permittivity (e) and permeability (l) pa-

rameter for the metamaterials is needed to match this crite-

rion.1 However, for single layer of planar metamaterial with

closely spaced continuous metal layer, the inhomogeneity

and anisotropy makes it complicated to determine effective eand l in absorber.18 Alternatively, the circuit model19 can be

used to investigate impedance matching as shown in Fig. 3.

The LC resonance of metamaterial is modeled by LCR reso-

nator, substrate by transmission line, and back metal by

ground at millimeterwave frequency. The coupling between

two resonators in dual band absorber is incorporated by mu-

tual inductive coupling, shown in Fig. 3(b). The circuit

model approximates the response of the absorber and cou-

pling between resonators in dual band absorber very well.

The samples are fabricated using standard optical photo-

lithography and metal liftoff process on 4 in. diameter polyi-

mide substrate with silicon wafer as a supporting substrate.

The titanium/gold metallization deposited by DC sputtering

with thickness of 30/200 nm is used for the patterning of res-

onators. Polyimide substrate is detached from silicon sup-

porting substrate and a thicker copper metal (1 lm) is

deposited at backside of the substrate as ground metal. Sam-

ples are diced with dimension of 45 mm� 45 mm for mea-

surement. For the measurement, we used a custom built

spectrometer as shown in Fig. 4. A frequency tunable back-

ward wave oscillator with range of 70-117 GHz is used as

signal source in the spectrometer. The calibration is per-

formed using metal reflector to determine the incident

power.

The measured peak absorptions for the single band

absorbers as shown in Figs. 5(a)–5(c) were 92% at 77.2 GHz,

94% at 94.8 GHz, and 99% at 109.5 GHz. The dimensions of

single band 77/95/110 GHz absorbers are L1¼ 333/272/

FIG. 4. (Color online) Schematic of the custom made backward wave oscil-

lator spectrometer setup used for the reflection measurement.

FIG. 5. (Color online) Measured (con-

tinuous lines) and simulated (dotted

lines) power reflection and absorption

coefficients of the single and dual band

absorbers: (a) 77 GHz absorber with

reflections for both parallel and perpen-

dicular polarizations, (b) 95 GHz, and

(c) 110 GHz absorbers. (d) Dual band

absorber. Inset showing microphoto-

graph of fabricated absorbers and image

of absorber wrapped around cylindrical

surface. These results show that absorp-

tion frequency can be tuned by changing

dimensions of the metamaterial unit cell.

264101-3 Singh et al. Appl. Phys. Lett. 99, 264101 (2011)

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245 lm, L2¼ 120/110/93 lm, L¼ 450/435/390 lm, W¼ 40/

35/34 lm, and S¼ 25 lm. Measured frequency response is

broadened in bandwidth attributed to the dimensional

changes of the unit cells in the fabrication process.1 This

dimensional variability also leads to the lower peak absorp-

tion than the simulated value (99.99%). The measured band-

widths for power absorption coefficient equal or greater than

80% is 4.3, 5.1, and 7.6 GHz for 77, 95, and 110 GHz absorb-

ers, respectively. Measured peak absorption increases for the

higher frequency absorbers. Also, measured base reflection

for 77 GHz absorber is below 80% and improves for the

absorbers at higher frequency. The scattering from the sur-

face could be responsible for this as surface morphology

improves with decrease in the unit cell size for higher fre-

quency absorbers. The measured peak absorption for the

dual band absorber as shown in Fig. 5(d) is 92% at 77 GHz

and 94% at 109.8 GHz with 80% absorption bandwidth of

4.8 and 6.6 GHz, respectively. The dimensions of dual band

77 /110 GHz absorber are L1¼ 355 lm, L2¼ 270 lm,

L¼ 450 lm, W1¼ 24 lm, W2¼ 33 lm, and S¼ 25 lm. Meas-

ured peak absorption frequencies are slightly lower than the

simulated value. However, measured frequency responses

are in close agreement with simulated results except for the

broadening of the bandwidth due to process variations.

In summary, we have implemented ultra thin metama-

terial absorbers at 77, 95, and 110 GHz frequency bands

on flexible substrate. Both single and dual band absorbers

at these frequencies are investigated. The total thickness of

the absorber is just 126 lm (almost 1/20th of the free

space wavelength). Measured frequency responses of the

absorbers are in very close agreement with the simulated

results. However, broadening of the bandwidth for meas-

ured response occurs due to process variations. Absorbers

in this frequency spectrum opens door for variety of appli-

cations in automotive radars, radomes, and point-to-point

wireless communications for both consumer and military

applications.

Authors acknowledge funding from Office of Naval

Research (ONR) through grant N00014-09-1-1075. Devices

were fabricated at Tufts Micro- and Nano-fabrication facility

(TMNF).

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